MEM Controller experienced operational error when assuming an altitude assignment had been accomplished; but in fact had not; resulting in the conflict.

Narrative

I was working both parallel finals combined. An A310 checked in on the right downwind for Runway 36R; and I descended them to 3;000 FT. A CRJ9 checked in on a right base for Runway 36R; and I descended them to 4;000 FT. A DC10 reported a 'slat handle' problem and broke off final for Runway 36R. I went back to verify that the A310 had checked in. Obviously they had. I instructed them to slow to 210 KTS at 6;000 FT. I thought I had stopped them at 5;000 FT for their descent. I took care of some other aircraft (I was working 8 aircraft; including an aircraft with a known problem; and watching a pointout). When I looked back at the A310 they were descending out of 4;800 FT for 4;700 FT. At the same time a fellow Controller asked me if those two were OK. I immediately turned the A310 to a 270 degree heading. I turned the CRJ9 to a 240 degree heading and descended them to 3;000 FT. I pointed out traffic to the CRJ9 twice; but the first time I issued 10 o'clock rather than 2 o'clock. My fellow Controller advised me of this. With the second and correct traffic call; the CRJ9 reported the traffic in sight; and I instructed them to maintain visual separation; but not before standard separation was lost. I have been working steady 6 day work weeks for some time. I believe that these work weeks have taken a toll and I felt mentally exhausted. I believe this could be a factor in me not making the correct traffic call the first time and my false belief that I had stopped the A310 at 5;000 FT when I hadn't.